Extend a scanned trunk with vertex blending

Tree trunks are often close to the camera and difficult to model which makes them an excellent candidate for photogrammetry. This page describes how to seamlessly extend a scanned trunk using vertex blending. The idea is to wrap the end of the scanned mesh around the beginning of a child branch and use vertex based alpha blending to hide the seam. A procedural canopy can be created off of the procedural child resulting in a complete model.

Note

The description of the process that follows assumes you are familiar with the basics of SpeedTree and is more of a guide to the general approach than it is a step-by-step tutorial.

Import the mesh and its textures

To use a 3D scan:

1. Import the mesh as a mesh asset.
2. Create a material asset that uses the textures associated with the scan. Be sure to associate the mesh asset with the material.

Note

Meshes must be a single model with a single material. Photogrammetry meshes are typically saved as single meshes with unwrapped UV data that correspond to a texture atlas.

Add the mesh to the model

Once you have assets in place, use a Mesh generator to make it part of your model.

1. Add a Mesh generator to the model.
2. Assign the material asset to the Mesh generator. The generator will automatically pick up the mesh asset via its association with the material asset.

Note

You can move or rotate a mesh manually by switching to node editing mode, clicking on the mesh in the scene, and using the gizmo to transform the mesh.

Add a wrap stitch

1. Select the Mesh generator.
2. Add a Wrap stitch (Add > Photogrammetry > Wrap stitch). You should see a Stitch generator and a Branch generator added to the trunk as pictured below.

Choose a good material for the child

Create a material for the child branch and apply it. This material should match the scanned trunk as closely as possible (all maps: color, normal, gloss, etc.).

Note

Make sure you tile the map on the child branch such that the bark pattern density matches that of the photogrammetry model. The quality of the blend will be greatly improved by a good match between the material on the photogrammetry model and the material/mapping it's being blended onto.

Put the stitch in a good spot

The stitch generates near the center of the mesh. If it doesn't generate in a valid position, you'll need to reposition it.

To reposition the stitch:

1. Switch to node editing mode.
2. Select either the stitch ring (if it's not valid) or the stitch geometry (if it is valid). 1. Move the stitch to the desired location. It should be placed near the end but not all the way to the end of the trunk as pictured below.

Note

Leave enough trunk geometry beyond the stitch location to be used in the blending process. The height of the blend region can be controlled in the Blend property group on the Stitch generator. Taller stitches require more trunk geometry beyond the stitch.

Edit the blend

Now that you have the stitch where you'd like it, it's time to make the blend region as seamless as possible.

To edit the blend:

1. Use the properties in the Blend:Opacity group to control the transparency of the trunk geometry in the stitch region. Vertices near the stitch line should be opaque while vertices near the top of the stitch will be transparent.
2. Use the Noise options to jumble up the effect.
3. Use the properties in the Blend:Geometry group to shape the trunk geometry in the blend region to the child geometry.

There will typically be a little bit of back and forth between blend settings, child positioning, child radius editing, and stitch positioning to get the stitch just right. When you're done, you should have something like the following.

Build out the rest of the model

Use traditional SpeedTree techniques to add a procedural branch structure and leaf canopy to the model. Start by adding child branches to the blended branch and go from there. Pictured below are a completed model and the corresponding generator hierarchy.

Note

It's often a good idea to add a few twigs or other decorations to the photogrammetry mesh using [Target generators]target-generator-properties.md) to further blend it in with the procedural geometry.

Best practices

The following list outlines a few things to keep in mind when using photogrammetry trunk meshes with vertex blending.

• Choose a reasonably smooth portion to place the stitch if you can.

• You can increase the Clearance property of a stitch to "eat" more of the trunk geometry above the stitch.

• Switch to node editing mode to move the trunk mesh, stitch, and branch until the stitch is as seamless as possible.

• You can increase the tesselation of the stitch. Doing so will cost extra polygons but it may make the stitch region blend more effectively.

• A single Stitch generator can create multiple stitches. Use this fact to stitch up several holes on a single mesh without having to have a unique generation editor configuration for each one.